Reinforced solder preform

ABSTRACT

There is disclosed herein a solder preform for use on printed circuit boards to provide reinforced solder joints thereon. The preform 70 comprises a solder body 72 made of solder, and a thin metallic member 30 attached to the solder body 72. The member 30 may be a metallic screen or foil having holes or perforations 32 therethrough, or may be a non-perforated metallic foil. A method for producing such reinforced solder joints is also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to printed circuit boards, andmore particularly to printed circuit boards having surface mountelectronic components.

2. Disclosure Information

One of the most common failure modes occurring in surface mount printedcircuit boards (PCBs) is solder joint cracking, as illustrated inFIG. 1. This is an electrical failure which is caused when the solderjoint 10 which electrically and mechanically connects a componenttermination 12 with its respective mounting pad 14 cracks due to thermalcycling stress induced because of the difference between thecoefficients of thermal expansion of the component/termination and themounting pad/substrate. Typically such cracks 16 initiate at the innerfillet surface 18 and propagate underneath the termination 12 and out tothe outer fillet surface 20, thus electrically separating thetermination 12 from its mounting pad 14. This problem is especiallypronounced in such environments as automotive applications, where thePCB must be able to perform in conditions anywhere between -40° C. and125° C.

Various approaches have been proposed for mitigating this problem, themost common being the use of higher melting point metal particles mixedinto the solder to impart more thermal fatigue resistance to the solder.However, it is often difficult to consistently control the standoffheight H using such an approach. Another approach is to use a leadedcomponent rather than a leadless one, but this has the disadvantage ofrequiring more board space and often a higher component cost.

It would be desirable, therefore, to provide some means for improvingsolder joint resistance to thermal cycling fatigue and cracking withoutthese disadvantages.

SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages of the prior art byproviding a solder preform for use on printed circuit board forproducing reinforced solder joints thereon. The preform comprises asolder body made of solder, and a thin metallic member attached to thesolder body. The thin metallic member is preferably made of asolder-wettable metal such as copper or nickel and may comprise anon-perforated metallic foil or a perforated metallic foil or screen.The present invention also includes a method for producing reinforcedsolder joints using such preforms.

It is an object and advantage that the present invention may be used toimprove solder joint resistance to thermal cycling stress.

It is a further advantage that the present invention may improve solderjoint resistance to thermal cycling stress without requiring the use ofparticle-filled solder or the use of leaded rather than leadlesscomponents.

These and other advantages, features and objects of the invention willbecome apparent from the drawings, detailed description and claims whichfollow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation view of a solder joint having a crack failuretherein according to the prior art.

FIGS. 2-4 are elevation views of various embodiments of a reinforcedsolder joint according to the present invention.

FIGS. 5-7 are perspective views of various embodiments of a thinmetallic member according to the present invention.

FIG. 8 is a schematic flowchart showing a method for producing a PCBhaving reinforced solder joints according to the present invention.

FIG. 9 is a graph of solder joint crack failure rate versus number ofthermal cycles, comparing conventional solder joints against thoseproduced according to the present invention.

FIGS. 10-11 are perspective views of solder preforms according to thepresent invention.

FIG. 12 is a schematic flowchart showing another method for producing aPCB having reinforced solder joints according to the present invention.

FIG. 13 is an elevation view of an electronic component having solderpreforms attached thereto according to the present invention

FIG. 14 is a schematic flowchart showing yet another method forproducing a PCB having reinforced solder joints according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, FIG. 2 shows a PCB 50 having reinforcedsolder joints according to the present invention. The PCB 50 comprises:a substrate 11 having mounting pads 14 arranged thereon; a surface mountcomponent 13 having terminations 12, with the component 13 beingdisposed on the substrate 11 with each termination 12 being registeredatop a respective mounting pad 14; a solder joint 10 connecting eachtermination 12 with its respective mounting pad 14; and a thin metallicmember 30 disposed within each solder joint 10 between its respectivetermination 12 and mounting pad 14.

The thin metallic member 30 may be a solid (i.e., non-perforated)metallic foil as shown in FIG. 5, or it may be a metallic screen or foilhaving holes or perforations 32 therethrough as shown in FIGS. 6 and 7.The member 30 has a melting point above the melting point of the solderused to form the solder joint 10 and above any other elevatedtemperatures to which the member 30 may be exposed during processing(e.g., reflow temperatures, which are typically higher than the meltingpoint of the solder used). Also, the member 30 should be constructed of(or at least coated with) a solder-wettable metal such as copper ornickel.

Preferably the thin metallic member 30 is substantially flat, isdisposed within the joint 10 generally parallel to the top surface ofits respective mounting pad 14, and is spaced apart from its respectivetermination 12 and its respective mounting pad 14, as illustrated inFIGS. 2-4. However, it is possible that the member 30 may be non-flat(e.g., corrugated or curved), may be oriented non-parallel to the topsurface of the mounting pad, and may rest against either the terminationor the mounting pad, and still be functional and within the scope of thepresent invention.

The size and shape of the thin metallic member 30 may be substantiallythe same as that of its respective mounting pad 14, or it may assumesome other shape and size. For example, the mounting pad may be a120-mil-wide by 90-mil-long rectangle, with the member 30 beingsubstantially the same size and shape. Alternatively, for such amounting pad, the member 30 may assume a circular shape having adiameter of 120 mils. The member 30 may assume other shapes and sizes aswell, but is preferably less than 5 mils thick.

If the member 30 is smaller than its associated mounting pad 14, it mayform a joint similar to FIGS. 2 or 3. In FIG. 2, the member 30 iscontiguous with both the inner and outer fillets 18/20, whereas in FIG.3 it is contiguous with only the inner fillet 18. It is also possiblethat the member 30 may "float" within the joint and not be contiguouswith either fillet, or that the member 30 is contiguous with only theouter fillet 20; however, it is preferred that the thin metallic member30 be contiguous with at least the inner fillet 18. If the member 30 islarger than its associated mounting pad 14, it may form a joint similarto FIG. 4. Here the member 30 is contiguous with both the inner andouter fillets 18/20. While both smaller-than-mounting-pad andlarger-than-mounting-pad members 30 will work, the latter is preferred.

The present invention effectively delays or avoids the onset of crackinitiation and slows down the propagation of any cracks that doinitiate. This is accomplished by essentially splitting the joint 10into two parts--that portion 10a of the solder joint above the thinmetallic member 30, and that portion 10b below. The thickness of eachpart 10a/10b is much less than the thickness H of a conventional joint,thereby making each portion 10a/10b stronger and more resistant to bothcrack initiation and crack propagation. The present invention isparticularly effective against stopping any mode I type cracks, whichtypically begin at the interior bottom edge of the termination 12 andpropagate more or less downward toward the mounting pad 14; if suchcracks do eventually initiate, the thin metallic member 30 stops thecrack from propagating any further downward when the crack reaches themember 30.

Actual test results with PCBs having reinforced solder joints accordingto the present invention indicate that use of such joints on a PCB candramatically improve the failure-free life thereof. Tests were conductedusing a 2512 leadless component resistor (LCR) on a conventional FR-4substrate having copper mounting pads thereon. The solder used was analloy of 62% Sn, 36% Pb, and 2% Ag. The average standoff height H was1.8 mils, using a thin metallic member 30 comprising a 0.8-mil-thick,120-mil-diameter circular copper screen having 400- to 500-mesh. Aconventional solder reflow process was used to form the solder joints.

Each PCB contained some 2512 LCRs with conventional, "control group"solder joints and other 2512 LCRs with reinforced, "test group" solderjoints, to assure that both groups experienced the same test conditions.Each PCB was subjected to either 1250 or 1500 thermal cycles between-40° C. and 125° C. At the end of 1000, 1250, and 1500 cycles, sampleswere cross-sectioned to determine how many crack failures occurred amongthe samples in each group. If at least one of the two terminations of anLCR was fully cracked, then that entire LCR was deemed as a failedcomponent. After 1000 cycles, no cracks were noticed in either group.After 1250 cycles, one out of four (1/4, or 25%) of the control sampleswere found to have failed due to solder joint cracking, while none outof six (0/6, or 0%) of the test samples had failed. After 1500 cycles,six out of sixteen (6/16, or 37.5%) of the control samples had failed,while only two out of twenty-four (2/24, or 8.3%) of the test sampleshad exhibited cracking. (It should be noted that the 1 out of 4 and 6out of 16 control samples were cracked completely through the joint,while the 2 out of 24 test samples that "failed" actually were onlypartially cracked and still functioned electrically.)

These results are illustrated in FIG. 9. These test results indicatethat using the reinforced solder joints of the present invention, thefailure-free life of the 2512 LCR was increased by roughly 25% (i.e.,from 1000 cycles up to 1250 cycles), and that the failure rate at 1500cycles was reduced roughly by a factor of at least 4.5 (i.e., from 37.5%down to only 8.3%).

A method for producing a PCB 50 having reinforced solder jointsaccording to the present invention is illustrated in FIG. 8 andcomprises the following steps. In a first step 100, a substrate havingmounting pads arranged thereon, a surface mount component havingterminations, and a thin metallic member for each termination areprovided. In a second step 200, a solder paste deposition is appliedatop each mounting pad. In a third step 300, a thin metallic member ispositioned atop each solder paste deposition. In a fourth step 400, thesurface mount component is placed on the substrate such that eachtermination is registered atop a corresponding thin metallic member. Ina fifth step 500, the solder paste is reflowed so as to form solderjoints connecting each termination with its respective mounting pad.When the solder paste is reflowed, it penetrates through any holes orperforations in the member and/or wicks around the peripheral edges ofthe member by capillary effect, thereby providing solder to the jointboth above and below the member. In the resultant joint, each thinmetallic member is sandwiched between its associated termination andmounting pad. An additional step 600 of depositing additional solderpaste atop each thin metallic member after the positioning step 300 andbefore the placing step 400 may be performed. As mentioned above, thethin metallic member should have a melting point above any temperatureto which the member is exposed during the reflowing step, or during anyother processing step.

An alternative approach to using deposited solder paste is to use solderpreforms instead. Such a preform 70 according to the present inventionis illustrated in FIG. 10, and would comprise a solder body 72 to whicha thin metallic member 30 is attached. The thin metallic member 30 herehas the same characteristics as the member 30 described above; forexample, it may comprise a non-perforated sheet, or a sheet or screenhaving holes or perforations 32 therein. The solder body 72 may assume agenerally rectangular wafer shape, as illustrated in FIG. 10, or it mayassume other shapes. Preferably the outer perimeter 78 of the member 30generally conforms with the outer perimeter 80 of the solder body 72,with each having a generally rectangular profile as viewed from above(i.e., as viewed from a point generally orthogonal to a top surface ofthe thin metallic member 30). The member 30 may lie atop the solder body72, as shown in FIG. 10, or it may lie within the solder body 72, asshown in FIG. 11.

Such preforms 70 may be constructed using various methods. One method isto first provide a large, flat, thin sheet of solder, such as may beformed by pouring molten solder into a mold of the desired shape andsize. Second, a large, flat, thin piece of non-perforated metal foil orperforated foil/screen may be attached to the large sheet of solder. Oneway of attaching the foil/screen to the solder is to sandwich themtogether while the solder is molten and allowing the sandwich to cool,thereby forming a solder bond therebetween. This step may be facilitatedby applying flux between the solder and the foil/screen, preferably byapplying a thin coat of flux to the surface of the foil/screen that isto be sandwiched in contact with the solder. Third, the sandwich maythen be stamped or diced so as to form a plurality of individual solderpreforms 70 cut to the desired size. This process results in preforms 70resembling that shown in FIG. 10. To produce preforms 70 similar to thatshown in FIG. 11, an additional step of pouring another layer of solderatop the foil/screen may be performed before the stamping/dicing step,thus forming a solder/member/solder sandwich.

A method for using the solder preforms to form the reinforced solderjoints of the present invention is shown in FIG. 12 and involves thefollowing steps. In a first step 701, a substrate having mounting padsarranged thereon, a surface mount component having terminations, and asolder preform for each termination are provided. In a second step 702,a first coating of flux is applied to the top surface of each mountingpad and/or to the bottom surface of each preform. In a third step 703, asolder preform is positioned atop each mounting pad with the firstcoating of flux interposed therebetween. In a fourth step 704, a secondcoating of flux is applied to the top surface of each preform and/or tothe bottom surface of each component termination. In a fifth step 705,the surface mount component is placed on the substrate such that eachtermination is registered atop a corresponding solder preform with thesecond layer of flux interposed therebetween. In a sixth step 706, thesolder and flux are heated so as to reflow the solder and form solderjoints connecting each termination with its respective mounting padwherein each thin metallic member is sandwiched between its associatedtermination and mounting pad. Alternatively, rather than applying fluxin steps 702 and/or 703, solder paste (i.e., solder particles suspendedin flux) may be applied instead; this may provide the added benefit ofproviding additional solder to each joint.

A modification of the above approach is to attach the solder preforms 70directly to each component termination 12, as shown in FIG. 13, prior toplacing the surface mount component 13 onto the mounting pads 14. Such acomponent 90 comprises a component body portion 15 having peripheralterminations 12 thereon. A tinned (i.e., solder-coated) member30--essentially a solder preform 70 as described above--is placed incontact with each component termination 12 and heat, pressure,ultrasonic vibration, etc., are used to create a solder bond betweeneach termination 12 and its associated thin metallic member 30.Substantially all of the solder 72 may be disposed between the member 30and the termination, as represented by joint A in FIG. 13, or it may bedisposed with a significant amount on both sides of the member 30 asrepresented by joint B. (Normally, only one of joint types A and B wouldbe used on a component; both are included in FIG. 13 primarily for thepurpose of illustration.) This kind of component 90 may be provided ontape reels just as conventional electronic components are, forsubsequent use in an automated pick-and-place machine. With the metallicmember 30 already attached to each component termination 12, no separatestep would be needed for placing a member 30 atop each solder pastedeposition, nor for placing a preform 70 atop a flux coating.

This kind of pre-membered component 90 could be used in a conventionalreflow process with little or no modification to the conventionalprocess, as illustrated in FIG. 14. In a first step 901, a substratehaving mounting pads arranged thereon and a surface mount componenthaving terminations are provided, wherein a thin metallic member issoldered to each termination as described above. In a second step 902, asolder paste deposition is applied atop each mounting pad. In a thirdstep 903, the surface mount component is placed on the substrate suchthat each member/termination is registered atop a corresponding mountingpad. In a fourth step 904, the solder paste is reflowed so as to formsolder joints connecting each termination with its respective mountingpad wherein each thin metallic member is sandwiched between itsassociated termination and mounting pad.

Various other modifications to the present invention will, no doubt,occur to those skilled in the art to which the present inventionpertains. For example, it is possible that the thin metallic member 30may be made from a plastic, ceramic, or non-solder-wettable metal havinga surface of solder-wettable metal plated or otherwise attachedthereabout. Also, the PCB 50 is not limited to conventional rigid FR-4epoxy-glass substrates, but may include flex circuits and other flexiblesubstrates. Furthermore, while the test results disclosed herein are fora 2512 LCR, other types of leadless (i.e., non-through-hole) surfacemount components may be used according to the present invention,including J-leaded, gull-winged, and other termination configurations.It is the following claims, including all equivalents, which define thescope of the present invention.

What is claimed is:
 1. A solder preform for use on printed circuitboards to provide solder joints thereon, comprising:a solder body madeof solder; and a thin metallic member having an outer surface made ofcopper or nickel attached to said solder body.
 2. A solder preformaccording to claim 1, wherein said thin metallic member has a meltingpoint above a melting point of said solder body.
 3. A solder preformaccording to claim 1, wherein said thin metallic member is a metallicscreen or foil having perforations therethrough.
 4. A solder preformaccording to claim 1, wherein said thin metallic member is anon-perforated metallic foil.
 5. A solder preform according to claim 1,wherein said thin metallic member is disposed on a top surface of saidsolder body.
 6. A solder preform according to claim 1, wherein said thinmetallic member is disposed within said solder body.
 7. A solder preformaccording to claim 1, wherein said thin metallic member is substantiallyflat.
 8. A solder preform according to claim 1, wherein an outerperimeter of said thin metallic member generally conforms with an outerperimeter of said solder body.
 9. A solder preform according to claim 1,wherein an outer perimeter of said thin metallic member and an outerperimeter of said solder body are each generally rectangular.
 10. Asolder preform for use on printed circuit boards to provide solderjoints thereon, comprising:a solder body made of solder; and a thinmetallic member having an outer surface made of a non-soldersolder-wettable material attached to said solder body, said thinmetallic member formed of a metallic screen or foil having perforationstherethrough.